Hot-cathode device



Sept. 11, 1928.

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Inventoj William D. Coolidge,

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Patented Sept. 11, 1928.

WILLIAM D. COOLIDGE, 0F SCHENECTADY, NEW YORK, ASSIGNOR TO GENERAL ELEC- TRIO COMPANY, A CORPORATION OF NEW YORK.

HOT-CATHODE DEVICE.

Application filed September 17, 1924. Serial No. 738,246.

The present invention relates to the construction of electrical discharge devices, in particular X-ray tubes, which contain a cathode operating at a high temperature, and in which the cathode is apt to become coated during operation by a. material having a higher electron emission than the material constituting the cathode.

For some purposes, it is advantageous to provide an X-ray tube with a target consisting of a metal of higher atomic number than tungsten, such as uranium or thorium, for example. For therapeutic purposes, X- ray output of such a tube is greater than that of a similar tube with a tungsten target.

In some cases as described in my prior Patent 1,323,836fit is desirable to provide a' quantity of finely divided thorium in an X- ray tube as a gas absorber. However, the presence in an X-ray tube, or other high voltage electrical discharge device, of metals of high electron emissivity, such as uranium or thorium whether present as a part of an electrode or without connection to the electric circuit of the device entails certain difficulties, especially when the cathode of the device is constituted of material of lower electron emissivity, for example when, in the case of the example given, the electrode con sists of tungsten.

Even when provision is made for cooling a uranium target, particles of uranium are carried to the heated cathode where they .locally increase the electron emission, thereby varying the'energy distribution on the focal spot andithe current in the X-ray tube in an irregular manner. This variation of the electron emission from the cathode is due to the characteristic of such metals as uranium in parting with electrons more readily than tungsten. This property is known as the work function also the electron atfinity of the material and is measurable as heat absorbed when electrons are emitted. .ln other words different materials require a diii'erent amount of energy to separate an electron from an emitting surface. Uranium and thorium have a lower work function than tungsten. This work function may be expressed in terms of potential difference in volts.

Certain other diiiiculties also arise due to the tendency for heated uranium and thorium to alloy itself with metals, such as copper` which are commonly used as a backing for the target surface.

In accordance with my invention I have provided hot cathode or thermionic devices containing in their construction materials having a lower Work function of electron emission than the cathode in which the above described diiculties are avoided. As will be more fully described hereinafter, the irregularities of electron emission, in so far as they cannot be avoided are prevented from affecting the operation of the device by a construction which creates a high space charge where the increased emission occurs, and thus prevents it from affecting the tube current.

In accordance with one modification of my invention the cathode is maintained at a sufficiently high temperature, except for the ends unavoidably cooled by the leads,to prevent the deposition of material, and electron emission from the ends is controlled by the construction of the focusing member in its relation to the cathode so as to shield these points and increase space charge at these regions.

In accordance with another modilication of my invention the cathode is so positioned with respect to the focusing device that electron emission from the entire cathode is controlled by space charge, so that for a given impressed voltage a given current is obtained regardless of variations of electron emission provided that the emission of electrons always exceeds a predetermined value for the given voltage.

The accompanying drawing shows in Fig. 1 an X-ray tube as illustrating one of the devices in which my invention will be embodied; Figs. 2. 3 and 4 are detailed views of different cathode structures whereby the results above described may be secured, and Fig. 5 is a sectional view of an anode having a target surface of metal of high atomic number. such as uranium or thorium.

The X-ray tube shown in Fig. 1 comprises as usual a bulb 2, a cathode 3 and an anode 4. As will be noted from Figs. 2, 3 and 4 the cathode comprises a filament adapted to be heated by conductors connected to an external screw base 5. These conductors are merely indicated by a dotted line at 6 as passing through a stem 7 projecting into the X-ray tube. The anode is cooled by 'water or other vfluid Vpassing' through the of uranium, and a backing plate 10 which preferably consists of copper. The copper tubes 8, 8 are connected by means of a spiral 11 which is brazed or otherwise placed in ood heat conducting relation with the bac plate 10. The target plate 9 when consisting of uranium is brazed to the back plate 10 by a binder layer 12 consisting of a copper-silver alloy, preferably an alloy consisting of 28 parts Acopper and 72 parts silver which is usually known as the eutectic alloy. In constructing the anode, the three members, that is the uranium-,face plate, the brazing alloy and the copper back plate, are heated in contact in a vacuum to the melting point of the bra-zing alloy. Under these conditions, if the time at the melt.-

`ing temperature is short, the alloy will bind the face plate and back plate without harmful allowa e of the uranium with the constituents o? the brazin alloy. This rocess may be carried out wit out melting t 1e coper, as the eutectic alloy has a materially ower kmelting point than copper.

The cathode 3 consists of a filament 13 of tungsten. It is surrounded by a' cylindrical member 14 which may consist of molybdenum and which preferably is provided with a flange 18, whereby positive ions which may be produced during exhaust of the tube are intercepted and prevented from injuring tube part-s back of the cathode. The cathode spiral A13 preferably is connected at one end to the tube 14 and at the opposite end to a central wire or rod 15 which projects beyond the cathode spiral and preferably sli htly beyond the focusing tube 14.

s the focusing cylinder is negatively charged by connection to the cathode, the electrons passing from the heated s iral 13 through the space Within-the focusmg cylinder 14 are subjected toa relatively high space charge, ing negative field in accordance with physical laws now well understood. If the cathode is situated far enough Within the focusing member as shown in Fig. 4 not all of the 4electrons emitted from the cathode can reach the anode at a given operating voltage of the X-ray tube. In other words by means of a construction such as shown in this figure the current in the X-ray tube is determined by the impressed voltage, and not by electron emission. Hence, a variation of electron emission, provided it is above a critical value, causes nol substantial change in the tube current. In the tubes thus constructed the deposition of uranium or thorium upon the filament will cause no change in the tube current. In order to prevent too sharp focusing the rod 15 projects slightly beyond or 1n other words to a retard` the periphery c lindcr 14 andy by its negative charge somew at broadens the focusing, slightly spreading the beam of electrons after its emergence from the cylinder 14.

In the construction shown in Fig. 2, the cathode spiral 13 is located close to the mouth of the focusing cylinder 14 and the electrons issuing from the main or spiral part of the cathode filament, therefore, are

not subjected to as .high space charge as in the construction of F ig..4. In this case the cathode filament is operated at a sulliciently high temperature to prevent uranium or thorium from remaining on the heated cathode filament. For example in an X-ray device 'a tungsten cathode when operated at about 2500 K. ork higher will not permit uranium from the anode to condense on .the highly heated .parts of the cathode spiral. The ends of the cathode are connected tothe cylinder 14 and to the rod 15 at points removed from the periphery of the focusing `cylinder 14. `These endsbeing in direct Athermal contact with metal parts are cooled by heat conduction to a temperature sufliciently low to permit the deposition of uranium or thorium. By the construction illustrated the electrons emitted fromthese parts of the cathode will be subjected to such a high space charge that .change of electron emission at these regions will have little or no effect upon the tube or space current.

In the construction shown in Fig. 3 the ends of the cathode spiral 13 are connected to points nearer the periphery of the focusing cylinder 14, and in this case flanges are provided upon the focusing cylinder 14 and the rod 15. 'As indicated in the drawing the flange 16 upon the cylinder 14 projects inwardly and thus shields the connection of the cathode filament to the.cylinder 14. The small disk-like flange 17 upon the rod 15 shields the opposite connection of the cathode filament. By means of these flanges the electron current from the cooled ends of the cathode is reduced to a low value, as the electrons emitted from these regions must bend past these negatively charged members in' order to pass to the anode. This deflection of thevelectrons causes a high space charge which reduces the electron current emanating from these regions which are liable to receive uranium or other material from the anlpde or wherever it may be present in the tu e.

Although my invention has been described with particular'reference to the construction and operation of X-ray tubes, it should be understood that my invention is not limited to X-ray tubes in its useful application.

In a high voltage rectifier as well as in an -ray device the presence of a material ofl high electron emissivity such as thorium or uranium, whether constituting part of the anode or being otherwise present, is an adeol vantage in improving the electron emissivity of the cathode, provided irregularities of emission are prevented as above described. As positive ion bombardment of the cathode tends to remove the layer of thorium, or other material of high emissivity, from the cathode surface, the operation of an electron device at high voltage would soon deactivate thel cathode if a renewal of the active surface were not provided for from a supply of this material in the device.

That I claim as new and desire to secure by Letters Patent of the United States, is

1. An electrical discharge device containing a cathode which is constructed to permit operation at a temperature of electron emission together with a material having a higher electron emissivity than said cathode and being capable of varying the electron emission at the cathode, the provision of means within the device for limiting the space current therein to a predetermined value for a given impressed voltage.

2. An Xfray device comprising a. bulb, a member therein comprising a metal having a higher electron emissivity than tungsten, a. tungsten cathode which is constructed to be heated by passage of current, and an electrostatic device so arranged about said cathode that an increase of electron current due to increased emission from volatilizable metal deposited upon the cathode is suppressed.

3. An X-ray device comprising an evac` uated bulb, an anode or target therein consisting of uranium, a cathode of tungsten which is adapted to be heated, and a focusing device said device being so arranged with respect to said cathode that an'increase ot electron current from the cathode due to the distillation of uranium thereon will be substantially prevented by space charge for a range of operating voltage.

4. An X-ray device having an anode or target consisting in part at least of a material having a lower work function than the material constituting the cathode, said device having an electrostatic focusing device so positioned with respect to the cathode that the space current in said device is lmaintained at a substantially constant value at a given operating voltage in spite of the deposition vof said anode material upon the cathode.

5. The method of preventing an increase of electron emission from a thermionic cathode by the deposition thereon of a ma terial of higher electron emissivity which consists in operating the main portion of said cathode at a temperature high enough to volatilize said material and preventing electrostatically an increase of current from said cathode at regions which unavoidably operate at a temperature at which said material can vary the emission of electrons.

6. An X-ray device having a thermionic cathode, portions of which are cooled by heat conduction to connected metal parts one of which parts comprises a focusing device, an anode or target consisting in part at least of a material having a lower work function,

than the material constituting the cathode,

and shielding means interposed between the anode and the cooled portions of the cathode.

7. An X-ray device having a thermionic cathode, an anode or target consisting in part at least of a material having a higher electron emissivity than the cathode, and means within the device for maintaining the space current therein substantially constant regardless of any deposition of the anode material upon the cathode.

8. In an electrical discharge device containing a cathode constructed to permit operation at a temperature of electron emission and an anode including a material of higher electron emissivity than the cathode and capable of varying electron emission of the cathode, the provision of means within the device for limiting the space current therein to a predetermined value for a given impressed voltage.

In witness whereof, I have hereunto set my hand this 16th day of September, 1924.

WILLIAM D. COOLIDGE. 

